Improving exercise tolerance, one of the major goals of physicians treating patients with cardiovascular disease, can be achieved either by improving cardiac output and blood flow to skeletal muscle, or by improving energy metabolism in skeletal muscle, or both. However, less is known regarding specific molecular pathways that might be approached therapeutically to improve exercise tolerance. In the past, we have demonstrated that the adenylyl cyclase type 5 (AC5) knockout (KO) mouse lives one third longer than wild type and is protected against aging induced cardiomyopathy, and the development of heart failure induced by either chronic catecholamine or pressure overload stress. The current project is based on these studies and our preliminary data demonstrating that AC5 KO mice exhibit increased exercise capacity. Our overall hypothesis is that AC5 is a critical enzyme affecting stress resistance and exercise capacity. The goal of this project is to examine mechanisms involved in AC5 inhibition, which could lead to a novel approach to improve exercise performance. There are two major hypotheses: HYPOTHESIS A: AC5 inhibition permits enhanced exercise performance due to improved limb blood flow through enhanced vasodilator mechanisms or angiogenesis, whereas improved cardiac function and cardiac output are less likely mechanisms. HYPOTHESIS B: AC5 inhibition permits enhanced exercise performance due to improved mitochondrial function and/or resistance to oxidative stress and/or improved glucose utilization. The implications for Public Health are clear: improving exercise tolerance will have broad significance for aging, heart disease, most other diseases and even for the young, healthy population.